Questions and Answers

The RWMA Manual and AWS C1.1 and others do not offer any published weld schedules for projection welding ring projections. A manufacturer of studs does offer some input on the subject. The RW 2110 stud has a full ring projection on a ¼-20 (6mm) threaded bolt. The following information is available to use as a starting point:

The answer to this will be weld nut manufacturing methods, cost, and final product design requirements. Nuts are generally formed with three to six projections on the one side. More projections would be used for higher strength applications. The shape of these projections also varies from points to rounded with various heights. The material being welded may also dictate the projection style and weld schedule speed or need for tempering if working with high strength steels. Nut manufacturing cost and final desired nut welded strength determine which style will be best for your application. Pointed and round projections will both make good projection welds. The question is which style and how many projections will be the most consistent in size, shape and final welded strength for your application.

In projection welding the design or shape of the part is used to make discreet individual point or contacts to concentrate the current during the welding process. In some applications a full ring projection or part edge is used to form a complete seal. The TO-5 transistor can is an example of a hermetic sealed device. The full edge of the bottom comes in contact with the lid and a full hermetic ring weld is formed. The actual current flow is controlled by the part design. The electrode is there to deliver force and current into the two work pieces and provide follow up force as the weld is made. Frequently Class 2 & 3 electrodes are used. In nut welding applications Elkonite facing electrodes are commonly used on the sheet steel side. The use of dissimilar electrode materials in projection welding is common. Normally it is done to improve mechanical wear or to resist the heat being generated.

Projection welding traditionally has been performed using AC controls and power supplies. The welders frequently have been traditional press welders and recently fixed station weld guns. both provide good alignment and can be designed for low inertia fast follow up systems. Rocker arm machines are not a good choice because they apply force in an arc and do not hold good alignment. In all cases fast follow up cylinders or servos are necessary for good quality.

With the advent of MFDC (Mid Frequency) controls and their accuracy these units are a logical player in projection welding. To make them suitable in this application, fast rise time technology has been developed because standard MFDC has a slow current rise. Another system that has shown some application for nut and stud welding is CD (capacitive discharge welders). This technology has been around for years. In the recent years CD has evolved into higher energy machines capable of nut and stud welding.

Projection welding schedules can be developed using standard spot welding schedules. For a single projection the weld time, current and force can be based upon available spot weld schedules. When welding multiple projections are being welded at one time and they are relatively far apart multiply the current times the number of projections. If the projections are close together and aligned, the total current will be reduced. Weld force is determined in the same manner. Weld time is the same no matter how many projections. Remember that in set up you always start on the cold side of power and work up to where you think you need to be to avoid damage to product and for the safety of people.

Weld schedules are available in the references below for some standard materials.

AWS and RWMA both publish spot welding schedules that can be used to develop a projection welding schedule. They also have published projection welding schedules for many materials.